The present invention relates to reducing noise in images captured from a camera.
Digital cameras acquire images using a CCD or CMOS sensor, which contains a two-dimensional array of sensor elements. Each CCD or CMOS element converts light into electrons that are subsequently converted into a (digital) signal. Each such element may correspond to a pixel in an image or frame captured by the camera. In addition to the desirable signal, invariably the image or pixel data that is output from the sensor or camera contains undesirable noise. There are many different types of noise originating from the sensor and camera device, for example, fixed-pattern noise, read-out noise, and shot noise. In order to accurately identify the underlying image content, the noise needs to be suppressed or otherwise removed.
Dark current is one cause of fixed pattern noise (“FPN”). Dark current is generally the result of spurious charge accumulating in each CCD or CMOS sensor element, even in the absence of light. Different amounts of spurious charge accumulate in different elements, hence, the fixed pattern noise results in spatially non-uniform patterns in the captured image. However, the spatial distribution is substantially time invariant, hence part of the dark current noise can be characterized as fixed-pattern noise. This type of FPN may be referred to as Dark Signal Non-Uniformity (“DSNU”). In addition to such DSNU noise, there is also dark current shot noise, which is spatially and temporally random noise on top of the dark current. Furthermore, the dark current noise is temperature-dependent, which makes the phenomenon less predictable in situations where temperature varies. In general, the dark current noise increases with increasing temperature.
Shot noise is fundamentally connected to the way photons arrive at detector elements. Photon shot noise is a spatially and temporally random phenomenon. Its noise variance generally depends on the signal strength. Read out noise may be considered as encompassing other random noise sources that are not generally dependent on signal strength, similar to additive white Gaussian noise. These may include amplifier noise, analog-to-digital conversion (ADC) noise, and several other types of noise.
It is desirable to reduce many of the different types of noise originating from the sensor and camera device, such as, fixed-pattern noise, read-out noise, and/or shot noise to accurately identify the underlying image content.
The foregoing and other objectives, features, and advantages of the invention may be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.